R-T-B sintered magnets whose primary phase is a Nd2Fe14B compound have been known as the best performance magnets among permanent magnets, and have been used in various motors, including voice coil motors (VCM) of hard disk drives and motors incorporated in hybrid vehicles, home electronics, etc. Since some or all of Nd may be replaced by a different rare earth element R and some of Fe may be replaced by a different transition metal element, the Nd2Fe14B compound will also be referred to as “R2T14B compound”. Note that some of B can be replaced by C (carbon).
Since the R-T-B sintered magnet has decreased coercivity at a higher temperature, irreversible degaussing occurs such that the coercivity decreases when exposed to a high temperature. To avoid the irreversible degaussing, maintenance of high coercivity is required even at high temperatures when the magnet is used for motors or the like. This cannot be achieved without increasing the coercivity at the normal temperature or decreasing the change in coercivity till a demanded temperature is reached.
It has been known that when Nd, which is the light rare earth element RL in a R2T14B compound phase, is replaced by a heavy rare earth element RH (mainly, Dy, Tb), the coercivity increases. Adding a large amount of heavy rare earth element RH to a source material alloy for the R-T-B sintered magnet has been considered to be effective in achieving high coercivity at high temperatures. However, when the light rare earth element RL (Nd, Pr) is replaced by a heavy rare earth element RH in the R-T-B sintered magnet, the residual magnetic flux density disadvantageously decreases although the coercivity improves. Also, the heavy rare earth element RH is a rare resource, and therefore, reducing the consumption of that element has been demanded.
In view of the above, in recent years, improving the coercivity of the R-T-B sintered magnet with a smaller amount of heavy rare earth element RH such that the residual magnetic flux density would not decrease has been studied. The present applicant already disclosed in Patent Document 1 that a heavy rare earth element RH, such as Dy, is supplied to a surface of a sintered magnet piece of a R—Fe—B alloy, and the heavy rare earth element RH is diffused into the sintered magnet piece (hereinafter, referred to as “depositional diffusion”).
According to the method of Patent Document 1, an R-T-B sintered magnet piece and an RH bulk of a heavy rare earth element RH need to be arranged in a treatment chamber such that they are spaced away from each other. Therefore, for example, the process for the arrangement is disadvantageously laborious. Further, since the supply of Dy or Tb is realized by sublimation, there is a probability that a long time is required to increase the amount of diffusion into the R-T-B sintered magnet piece and achieve higher coercivity.
In view of the above, the present applicant disclosed, in Patent Document 2, a manufacturing method of an R-T-B sintered magnet, including the step of providing R-T-B sintered magnet pieces, the step of providing RH diffusion sources which are made of a metal or alloy of a heavy rare earth element RH (at least one of Dy and Tb), the step of loading the R-T-B sintered magnet pieces and the RH diffusion sources into a treatment chamber such that the R-T-B sintered magnet pieces and the RH diffusion sources are relatively movable and can be in the vicinity of each other or in contact with each other, and the RH diffusion step of performing a heat treatment at a temperature not less than 500° C. and not more than 850° C. for not less than 10 minutes while continuously or intermittently moving the R-T-B sintered magnet pieces and the RH diffusion sources in the treatment chamber.
According to the method of Patent Document 2, the RH diffusion sources are in the vicinity of or in contact with the R-T-B sintered magnet pieces even at the temperature of not less than 500° C. and not more than 850° C. Therefore, the heavy rare earth element RH is supplied from the RH diffusion sources and can be diffused into the R-T-B sintered magnet piece through the grain boundary.
The present applicant also disclosed, in Patent Document 3, a manufacturing method of an R-T-B sintered magnet, including the step of providing an R-T-B sintered magnet pieces in which the amount of R, which is defined by the content of a rare earth element, is not less than 31 mass % and not more than 37 mass %, the step of providing RH diffusion sources which include a heavy rare earth element RH (at least one of Dy and Tb) and Fe in the proportion of not less than 30 mass % and not more than 80 mass %, the step of loading the sintered magnet pieces and the RH diffusion sources into a treatment chamber such that the sintered magnet pieces and the RH diffusion sources are relatively movable and can be in the vicinity of each other or in contact with each other, and the RH diffusion step of heating the sintered magnet pieces and the RH diffusion sources to a treatment temperature of not less than 700° C. and not more than 1000° C. while continuously or intermittently moving the sintered magnet pieces and the RH diffusion sources in the treatment chamber.
According to the manufacturing method disclosed in Patent Document 3, the heavy rare earth element RH can be diffused into the R-T-B sintered magnet piece (the magnet before execution of the RH diffusion step) within a short time period, such that HcJ can be improved without decreasing Br. Further, even though the RH diffusion step is carried out in a wide temperature range of not less than 700° C. and not more than 1000° C., the R-T-B sintered magnet pieces and the RH diffusion sources would not cause fusion, and the heavy rare earth element RH can be diffused into the R-T-B sintered magnet piece.
The entire contents of Patent Documents 2 and 3 are incorporated by reference in this specification.